1. Field of the Invention
This invention relates to a method and apparatus for removing trace amounts of oxygen and nitrogen from crude argon and to particular apparatus useful in said removal wherein the crude argon is generally obtained by cryogenic distillation of air.
2. State of the Art
Argon is a useful inert gas which has many application such as in light bulbs, in the welding of metals, as inert atmosphere for steel production as well as in various electronic industries, and the like. A major source of argon is atmospheric air, about 1% of which is argon.
Commercially, argon is produced as a valuable by-product in cryogenic air separation plants for producing oxygen and nitrogen. Crude argon produced cryogenically usually contains trace amounts of nitrogen (100 ppm to 1% by volume) and appreciable quantities of oxygen (100 ppm to 7% by volume). This crude argon stream must be purified to reduce nitrogen and oxygen before it is suitable for use, particularly as an inert gas. Because of the proximity of the boiling point of argon (87.28.degree. K.) and oxygen (90.19.degree. K.), distillative separation of argon and oxygen in particular is very difficult and energy intensive.
Heretofore, oxygen has been removed from crude argon streams by catalytic reduction to water with excess hydrogen over platinum catalyst beds, referred to herein as the deoxo process, followed by drying to remove the water and then by dual pressure distillation to remove nitrogen and excess hydrogen. See, for example, R. E. Latimer, "Distillation of Air," Chemical Engineering Process, pp. 35-59, February 1967, which illustrates a typical scheme.
Although argon streams purified by this method usually contain only ppm levels of nitrogen, oxygen and hydrogen, the process does have significant drawbacks. First, the hydrogen used in conventional cryo/deoxo processes is expensive. For example, for a crude argon stream containing only about 2.8% oxygen, about 3 mols of hydrogen are consumed for each ton of argon processed. At eight dollars per thousand standard cubic feet of hydrogen cost, the oxygen removal cost is $9.20 per ton of argon for hydrogen consumption alone. Further, hydrogen is not always conveniently available in many parts of the world.
Another shortcoming of the cryo/deoxo process for purifying argon is that the water produced from the deoxo reaction must be removed completely before the argon is fed to the final cryogenic distillation column. This requires feeding the argon stream through a dryer preliminary to the cryogenic distillation. Capital and operating costs associated with this additional step add significantly to overall cost.
Further, the excess hydrogen introduced to remove the oxygen in the first place must itself be removed and recovered before a pure argon stream can be produced. This adds further to the complexity and cost of the overall design and operation of the process.
A more recent concept for purifying crude argon is disclosed in U.S. Pat. No. 5,035,726, assigned to Air Products. The patent discloses deoxygenating crude argon by use of an oxygen ion transporting membrane, i.e. a solid electrolyte oxygen concentration (SEOC) unit, and then removing nitrogen by returning the oxygen-depleted argon stream to an argon/nitrogen cryogenic distillation unit. Such an argon purification system must necessarily be located close to an argon/nitrogen/oxygen production facility, which is usually a large plant.
Also, the Air Products system involves heating cooled crude argon to the operating temperature of the SEOC unit, which is typically about 700.degree. C. and above, then cooling the oxygen-depleted argon and refrigerating it to a cryogenic distillation temperature (.about.87.28.degree. K. or .about.-185.degree. C.). Heating and Cooling of the argon gas to such temperature extremes tends to be energy inefficient. Further, the Air Products system contemplates Bi.sub.2 O.sub.3 as an electrolyte, which is generally unstable under conditions of low O.sub.2 concentration, tending then to reduce to electronically conducting bismuth suboxides or even bismuth metal.
Other techniques for purifying argon gas streams have also been suggested. For example, U.S. Pat. Nos. 4,144,038 and 4,477,265 suggest separating argon from oxygen using aluminosilicate zeolites and molecular sieves. Such processes trade argon recovery for purity.
U.S. Pat. No. 4,230,463 suggests using polymeric membranes such as polysulfones, polysiloxanes, polyaryleneoxides, polystyrenes, polycarbonate, cellulose acetate and the like for separating pairs of gases such as hydrogen and argon and polymeric membranes such as polysulfones have been suggested for the removal of oxygen from argon. Studies of hybrid processes involving cryogenic distillation and membrane separation have been reported as, see, for example, Jennings, et al., "Conceptual Processes for Recovery of Argon with Membranes in an Air Separation Process," American Institute of Chemical Engineers, 1987 Summer National Meeting, and Agrawal, et al., "Membrane/Cryogenic Hybrid Scheme for Argon Production from Air," American Institute of Chemical Engineers, 1988 Summer Meeting in Denver, Colo. Selectivity and recovery in such hybrid schemes has been rather poor. Much of the argon permeates with oxygen through membranes and must be recycled to crude argon distillation columns.
Another technique for removing trace amounts of oxygen and other impurities from argon has involved lithium containing organic resins. Generally, the lithium is not regenerated. The lithium converts to lithium oxide.
Therefore, there is a need in the industry for an improved process for purifying crude argon produced by cryogenic air separation.